The present invention relates to an automatic gain control circuit, a receiving apparatus incorporating the circuit, an automatic gain control method for the receiving apparatus and a recording medium on which a program for causing the automatic gain control method to be executed. More particularly, the present invention relates to an automatic gain control circuit arranged to commonly use an automatic gain control loop to reduce the size of the circuit so as to prevent enlargement of the mounting area of a circuit substrate and power consumption in the circuit and a receiving apparatus incorporating the circuit. More particularly, the present invention relates to an automatic gain control method for a receiving apparatus with which elongation of time required to complete a process and enlargement of power consumption can be prevented when a software program is used to automatically control gains of a plurality of receiving systems and a recording medium.
As a conventional automatic gain control circuit for a receiving apparatus, an automatic gain control circuit having a structure, for example, as shown in FIG. 10, is known. Referring to FIG. 10, the conventional automatic gain control circuit comprises a variable gain amplifier 11, a demodulating portion 12, an A/D converter 13, a level detector 14, an averaging portion 21, an adder 22 for calculating difference in a converged value, a multiplier 23 for controlling a loop gain, an adder 24 of an integration circuit, a latch circuit 25 of the integration circuit, a calculating portion 26 and a D/A converter 27.
In the foregoing conventional automatic gain control circuit, when received signal Ri has been input, the received signal Ri is amplified by the variable gain amplifier 11, and then demodulated by the demodulating portion 12. Then, the received signal Ri is converted into a digital value by the A/D converter 13 so as to be demodulated output Rd. The level of a portion of the demodulated output Rd is detected by the level detector 14, and then fetched by an automatic gain control loop.
Data subjected to the level detection is averaged for a predetermined time by the averaging portion 21. Then, convergence to the input of the A/D converter 13 is realized by calculating the difference from a predetermined target level A by the adder 22 for calculating difference in a converged value. Then, the multiplier 23 for controlling a loop gain multiplies loop gain control value B in the automatic gain control circuit. The output of the multiplier 23 is, as change from preceding data, input to the integration circuit composed of the adder 24 and the latch circuit 25. Thus, integrated data is latched in the latch circuit 25 at the timing of the latch timing control value D. Integrated data of the integration circuit is converted into data corresponding to a control voltage for the variable gain amplifier 11 by the calculating portion 26. The D/A converter 27 converts data into an analog voltage so as to be fed back as the control voltage for the variable gain amplifier 11.
Hitherto, when the gains of a plurality of receiving systems are automatically controlled by using the automatic gain control circuit structured as shown in FIG. 10, the automatic gain control loops must be formed by the number which is the same as the number of the receiving systems. That is, when one automatic gain control loop shown in FIG. 10 is used to control a plurality of receiving system, for example, two receiving systems, the automatic gain control circuit shown in FIG. 10 is provided for each of the receiving systems, as shown in FIG. 11.
As described above, the conventional automatic gain control circuit and the receiving apparatus incorporating the circuit above are used to automatically control the gains of n (n is a positive integer not smaller than two) receiving systems. When the automatic gain control loop is structured by hardware, that is, constituted by electronic elements, the size of the automatic gain control loop block is enlarged to n times the size required when one receiving system is controlled. Thus, there arises a problem in that the mounting area of the circuit substrate is enlarged or the power consumption in the circuit is enlarged.
When a DSP (Digital Signal Processor) is employed in the automatic gain control loop to use a software program to automatically control the gains of n receiving systems, there also arises a problem in that time required to complete the automatic gain control process is elongated and, therefore, power consumption is enlarged.